1. Field of the Invention
The present invention relates to a multi-colored image forming apparatus which develops an image in multiple colors through a plurality of developing devices, transfers this image developed in multiple colors on an image receiving medium and fixes the transferred image on the image receiving medium as a permanent image and, more particularly, to a multi-colored image forming apparatus which has an improved transfer mechanism for transferring a developed image on an image receiving medium.
2. Description of the Related Art
A multi-colored image forming apparatus which has been disclosed in the Japanese Laid-open Patent Publication No. P02-105174 has been well known. That is, a method to form a developed image on a photosensitive drum and repetitively transfer a developed image in each of multiple colors on an image receiving medium has been put to practical use. Further, as an image receiving medium carrier, such laminate insulators as plastic and the like, which are capable of holding charge, were so far used and an electric field for transferring images was formed by charging this insulator by a corona charger. However, there was such a problem that ozone poisonous to the human body would be generated if the transfer electric field was formed as described above.
In order to solve this problem, as shown in the Japanese Laid-open Patent Publication No. P03-287861, an image transfer device which uses a semi-conductive sheet as an image receiving medium carrier has been considred. When such a semi- conductive sheet is used as an image receiving medium carrier like this image transfer device, a satisfactory image transfer at a low bias voltage below 2 kV becomes possible and the necessity for providing a discharging device for the image receiving medium carrier can be eliminated as the self-discharging is possible. In addition, there is such a merit as no ozone will be generated at all.
However, in case of this transfer device, the image transfer margin of a half-tone image with a small quantity of toner applied is narrower than the transfer margin of a solid image with much toner applied, that is, the range of transfer bias voltage under which an image can be transferred satisfactorily. Therefore, there was such a problem that transfer efficiency of transfer bias voltage for a half-tone image changed larger than that of a solid image and a half-tone image cannot be transferred satisfactorily under the same transfer bias voltage if electric resistance of an image receiving medium changed by change in environmental conditions such as temperature and humidity.
In particular, when multiple transfers were performed by providing a bias voltage applying part to each of the transfer portion of an image carried in a multi-colored image forming apparatus which has a plurality of image carriers, transfer efficiency of a hlaf-tone image changes remarkably with the increase of the number of transfers. This is because a size of the transfer electric field is changed in the direction of multiple transfers by the charge of an image receiving medium or the effect of developers which were already transferred on the image receiving medium if the image was transferred by applying the same bias voltage despite of a plurality of the surfaces of image carriers being charged to the equal potential. Therefore, there was such a problem that when the same transfer bias voltage was applied to a plurality of bias voltage applying parts, the color reproducibility of the colored image obtained is worse and the multi-colored image forming of good quality cannot be performed.
Because of such problems as described above, in order for stably performing a satisfactory multi-colored image transfer, it was so far necessary to provide environmental sensors to sense temperature humidity or a transfer bias power source which is capable of automatically varying output voltage. As a result, a multi-colored image forming apparatus became complicate in the structure and caused in cost increase.
Further, the Japanese Laid-open Patent Publication No. P53-93031 disclosed a multi-colored image forming apparatus which forms images developed in different colors on a plurality of photosensitive drums and sequentially transfers these colored images on a transfer paper being conveyed by a belt, thus obtaining a multi-colored image. In this multi-colored image forming apparatus, transfer rollers have been arranged by pressure fitting them closely to the back of the belt at transfer positions to pressure fit the belt to the photosenstitive drums, and voltage of polarity reverse to the developers are applied to these transfer rollers. Voltage to be applied to the transfer rollers are made higher gradually in the transfer sequence. That is, when transferring the images developed in different color developers sequentially, transfer efficiency is increased by applying higher voltage to the second color than the first color, to the third color higher than the second color and so on. In this embodiment of the Publicaiton, bias voltage of 2.5 kV, 3.0 kV and 3.5 kV are applied independently to the transfer rollers for the first, second and third colors, respectively.
However, as higher bias voltages are applied independently to respective transfer rollers sequentially, a plurality of power sources become necessary. As a result, there is such a problem that a power supply occupies a large space in the multi-colored image forming apparatus and the apparatus would become expensive.